DGM-34 (Benson), v. 2.0 GEOLOGIC MAP OF THE BENSON...
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Arizona Geological Survey DGM-34 (Benson), v. 3.0 Arizona Geological Survey DGM-34 (Benson), v. 2.0 Qi 1 Qys Tsd Yg Qi 2 Qi 1 Qi 1 Qy 2 r Qi 1 Qi 1 Qy 1 Qi 2 Qi 1 Qys Qi 2 Qy 1 Qi 2 Qy 3 Tsd Tsd Qy 1 Qi 2 d Qi 2 Qi 1 Qyi Qi 2 Qyi Qi 2 Qy 2 Tsd Qi 2 QTcs Tsdm Qi 2 QTcs Qys Qi 1 Qi 2 r Qi 2 Qy 2 Qyi Qy 2 Qi 1 Qys d KJs Qy 3 Qyi Qyi Qi 1 Qy 2 Qi 2 Qi 2 Qy 1 Qi 1 Qy 1 Qi 1 Qy 1 Qi 1 Tsd Qi 1 Qi 2 Qyi Qyi Qy 1 Qy 3 Tsd Qyi Qy 2 Qy 1 Qi 2 Qyi Qi 2 Tsd Qi 2 Qy 1 Qi 1 Qy 1 Qy 1 Qys Qy 1 Tsd Qi 2 KJs Qi 2 Qi 1 Qy Qy 1 Tsd Qy 1 Qi 2 d Qyi Qi 2 Qyi Qy 2 Qi 3 Qyi Qy Qys Qi 2 Tsd Qys Qyi QTcs Qi 1 Qy 1 Qys Qi 2 Qi 2 Qy 1 Qy 1 Qi 2 Qy 3 r Qyaf Qi 2 Qy 1 Qi 2 Qys Qi 2 Qi 2 Qys Qi 2 Qyi Qi 2 Qyi Qi 2 Qyi Qy 1 Qi 2 Qi 2 Tsd Qi 2 Qy Qy 1 Qi 1 Tsd Tsd Qi 2 Qy 3 r Qi 2 Qy 2 Qyi Tsd Qi 2 Qys Qi 2 Qyaf Qi 2 Qi 1 Qys Qi 2 Qi 2 Tsd Tsd Qy 2 r KJs Qi 2 Qi 2 Qi 2 Qi 2 Qy 2 Qyi Qy 1 Qy 2 r Qi 2 Qys Tsd Tsd Tsd Qi 2 Qy 3 Qy 2 Qys Qi 2 Qi 2 Qi 2 Qyi Qi 1 Qi 2 Tsd Qi 2 Qi 2 Qi 2 Qi 2 Qys Tsd Qyi Qy 1 Qy 1 Qi 2 Qyi Qy 3 r Qi 3 r Qi 2 Qy c Qy 2 Qy 1 Qi 3 Qi 3 Ygm Qyi Qys Qi 2 Qys Qi 1 Tsd Qys Qy 2 r Qys Tsd Qi 2 Qi 1 Qi 2 Tsdm Qi 1 Qy 1 Qi 1 Qi 2 Qi 2 Qi 2 Qy 2 Qys Qi 2 Qi 2 Qy 1 Qyi Qi 2 Qy 1 Qi 2 Qi 2 Qys Qi 1 Tsd Qys Tsd Qi 1 Qy 2 QTc Qy 3 r Tsd Qi 2 Tsd Qyi Qys Qi 2 Qyaf Qy c Tsd Tsd Qy Tsd Tsdl Qys QTcs Tsd Qy 1 Qi 2 Qy c r Qi 1 Tsd Qi 2 Qy 2 Qy 2 Qi 2 Qi 2 Qys Qi 2 Tsd Qi 2 QTcs Qy Qi 2 Yg Qi 2 QTcs Qy 2 r Qi 2 Qys Qi 1 Yg Qy 1 Qi 2 Qi 1 Qyi Qi 1 Qys Qys Qi 2 Qi 2 Qi 1 Qys Yg Qi 2 Qy 2 Tsd Qi 2 Qi 2 Yg Yg Qi 1 Qi 2 Qi 2 Qy Qyi Qi 2 Tsd Tsd Yg Qi 1 Qys Qyi Qy 1 Tsd Qyaf Tsd Yg Qi 2 Qy c Qys Qi 1 Qi 1 Qy 1 Qi 2 Qi 2 Qys Qi 2 Qi 1 Qyi Qi 2 Qi 1 Tsd Qi 2 Qys Qi 2 Yg Qi 2 Tsd Qi 1 Qi 2 Qi 2 Qi 2 QTa QTa QTa Qy 1 Qi 2 Qi 1 Qy 3 r Qi 2 Qy 3 r Qy 1 Qi 2 Qi 2 Qi 2 Qy 1 Qy Qi 2 Qy 3 Qi 3 Qy 1 Tsd Tsd Tsd Qy c Qy c Qy c r Yg Qi 2 Qy 1 Tsd Qyi Qy Tsdl Qy 1 Tsd Qy Qy c r Qy 1 Qy 1 Qy 2 Qy c Tsd Qi 2 Tsd Qy Qy 2 Qy 2 QTcs Yg Tsd Qyi Qy Qy 1 Qy Yg Tsd Qi 3 Tsdl Qi 3 r Qi 1 Qy 1 Qyi Qy Tsd Qy Qy 1 QTcs Yg Tsd Qy 1 Qy 2 Qi 2 Qi 2 Qi 2 r Qy 2 QTcs Qi 1 Qy Yg Qy Qy 3 Qyi Qi 1 QTcs Yg Qy 2 Qy 1 Qy 2 Tsd Qy 3 r QTcs Qy Qy Qy Qy 1 Qi 2 Qi 3 Qy 1 Qys Qy 1 Qy 1 Qi 3 KJs Qy 2 Tsd Tsd Qy 1 Qy 2 Qy 1 Qy 2 Tsd Tsd Ygm Qyi Qi 2 Qy 1 Qy 2 KJs Qy 2 Qy 1 Qy 2 Qys Qy KJs Qy 2 Qi 2 Qy Qy c Qi 2 Qy 2 Qy 1 Qy 1 QTc Qi 3 r Tsd Tsd Qy 1 Qy 3 Qy 2 Tsd KJs d Qy 1 Ygm Qy 1 Qy Ygm Qy 2 Qi 2 Qy Tsd Qy c Qi 1 Qy 1 Qyi Qys QTcs Qi 2 Qi 3 r Qi 2 Qy Yg Qy 1 Qi 2 Qy Qi 3 r Qy 1 Qy 1 Qy Qy Qy 1 Tsd Qy 1 Qyi Qys Qy 1 QTcs Qy Qi 3 r QTcs Qy 1 Qy c Qy Qy 2 Qi 2 Tsd Qy 1 Qi 3 r KJs Qy 2 r Qy 1 QTc Qy 3 Qy Qyi Qi 3 Qi 2 Qi 2 Qy 2 Qi 3 r Qi 2 Qy Qy 2 Qy 2 Qy 2 Qy Qi 3 Qi 2 Qi 2 QTcs Qi 1 Qy Qyi Tsd Qy Qy 1 Tsdl Tsd KJs Tsd Qys Qy 1 Qy 1 Tsd Qys Qi 2 Qy 1 Qi 3 r Yg Qy 2 Qy Qy 2 Qy 2 Qy Qi 1 Qy 1 Qy 2 Qy 2 Tsd Tsd Qy Qi 3 r Qy Qy 1 Qi 3 Tsd Tsd Qy Qi 2 Qi 3 Qi 1 Qy c Tsd Qy c Tsd Qy 1 Qy 2 Qy 1 Ygm QTcs Qy 1 Qi 3 r Qi 2 Tsd Qy 1 Qyi Qi 2 Qy 2 Yg Qyi Tsd Qi 2 Tsd Qi 1 Tsd Qy Qy 1 Tsd Qy Tsd Qi 1 Qyi Qy Qyi Qy Qyi Qi 3 Qy 1 Qi 2 Tsd Qy Qi 3 Qy 2 Qy 1 Tsdl Tsd Qy 1 Qy 1 Qy QTcs Qy 2 Qi 2 Qi 3 Tsd Qi 3 r Qy 2 Qi 3 r Qi 2 r QTc QTcs Tsd Qy 1 Qi 2 r Qy 1 Qy Tsd Qi 2 Qy Qy 2 Qy Qy 2 Qi 1 Qi 2 Tsd Qy Qi 2 Qy 1 Qyi Tsd Qyi Qi 2 Qy 1 Qi 1 Qy Qyi Qys Qy Qi 3 Qy 1 Yg Qyi Qi 3 Qy 1 Qy 1 QTa QTa QTa QTa QTa Qy 3 r 5 59000 5 60000 5 61000 5 62000 5 63000 5 64000 5 65000 5 66000 5 67000 5 68000 5 69000 5 70000 5 71000 5 59000 5 60000 5 61000 5 62000 5 63000 5 64000 5 65000 5 66000 5 67000 5 68000 5 69000 5 70000 5 71000 35 27000 35 28000 35 29000 35 30000 35 31000 35 32000 35 33000 35 34000 35 35000 35 36000 35 37000 35 38000 35 39000 35 40000 35 27000 35 28000 35 29000 35 30000 35 31000 35 32000 35 33000 35 34000 35 35000 35 36000 35 37000 35 38000 35 39000 35 40000 110°15' 110°15' 110°17'30" 110°17'30" 110°20' 110°20' 110°22'30" 110°22'30" 32°0' 32°0' 31°57'30" 31°57'30" 31°55' 31°55' 31°52'30" 31°52'30" 2006 MAGNETIC NORTH DECLINATION ^ 10½º MN Arizona Geological Survey 1955 E 6th St Tucson, AZ 85721 (520) 621-2352 www.azgs.arizona.edu Topographic base from USGS 1:24,000 scale quadrangle series. North American Datum of 1983 (NAD83). Projection and 1,000-meter grid: Universal Transverse Mercator, zone 12. 1 0 1 0.5 Miles 1 0 1 0.5 Kilometers 0 1000 2000 3000 4000 5000 Feet SCALE 1:24,000 CONTOUR INTERVAL 20 FEET Map Symbols Faults fault, accurate fault, concealed fault, concealed, hypothetical @ Contacts accurate contact approximate contact scratch contact concealed contact Youberg and Shipman Skotnicki Ferguson Youberg and Skotnicki Cook and Youberg 110°15' 110°15' 110°17'30" 110°17'30" 110°20' 110°20' 110°22'30" 110°22'30" 32°0' 32°0' 31°57'30" 31°57'30" 31°55' 31°55' 31°52'30" 31°52'30" Mapping Responsibility GEOLOGIC MAP OF THE BENSON 7½' QUADRANGLE, COCHISE COUNTY, ARIZONA Ann Youberg, Steven J. Skotnicki, Charles A. Ferguson, Joseph P. Cook and Todd C. Shipman Arizona Geological Survey Digital Geologic Map 34 (DGM-34), version 3.0 September 2019 1:24,000 scale BISBEE BENSON COCHISE DOUGLAS TOMBSTONE SIERRA VISTA § ¨ ¦ 10 £ ¤ 80 £ ¤ 92 £ ¤ 186 £ ¤ 181 £ ¤ 82 £ ¤ 90 £ ¤ 191 109°0' 109°15' 109°15' 109°30' 109°30' 109°45' 109°45' 110°0' 110°0' 110°15' 110°15' 110°30' 110°30' 32°15' 32°15' 32°0' 32°0' 31°45' 31°45' 31°30' 31°30' Benson 7.5' Quadrangle is located in western Cochise County. LAND MESCAL BENSON SAINT DAVID APACHE PEAK MCGREW SPRING SAN PEDRO RANCH GALLETA FLAT EAST GALLETA FLAT WEST HABERSTOCK HILL 110°7.5' 110°7.5' 110°15' 110°15' 110°22.5' 110°22.5' 110°30' 110°30' 32° 32° 31°52.5' 31°52.5' Adjoining 7.5' Quadrangles Location Index Map Quadrangle Location Shown in Blue Cochise County Mapped Area Shown in Blue Unit Correlation Citation for this map: Youberg, A., Skotnicki, S.J., Ferguson, C.A., Cook, J.P., and Shipman, T.C., 2019, Geologic Map of the Benson 7½' Quadrangle, Cochise County, Arizona: Arizona Geological Survey Digital Geologic Map 34 (DGM-34), version 3.0, 1 sheet, layout scale 1:24,000. (also available in Adobe pdf format on CD-ROM) Not to be reproduced for commercial purposes Research supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program, under USGS award #02HQAG0016. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. government. The Arizona Department of Water Resources provided funding for map revisions depicted in DGM-34, version 2.0. Acknowledgements: Genevieve Pearthree was the cartographer for DGM-34, version 2.0. Map Unit Descriptions Other units Plowed areas - historically or actively plowed fields, irrigated pastures, and other lightly disturbed ground. Historical river terrace deposits - Terrace deposits that occupy elevations from 1 to 2 meters above Qycr deposits and are inset below the pre-incision historical floodplain. These surfaces are generally planar but exhibit bar and swale microtopography. Although no soil development is present, dense grasses and small mesquite trees abound. Sediments composing these deposits are poorly sorted silt, sand, pebbles and cobbles. Pebbles and cobbles are well-rounded to sub-angular. Trough crossbedding, ripple marks, and stacked channel deposits viewable in cross-section indicate deposition in a low to moderate energy braided stream environment. These deposits are prone to flooding during extreme flow events, and undercutting and rapid erosion of Qy3r surfaces is possible during lower flow events. Qy3r Latest Holocene to historical river terrace deposits - Deposits associated with the floodplain that existed prior to the early historical entrenchment of the San Pedro River (Hereford, 1993; Huckleberry, 1996; Wood, 1997). Qy2r deposits are associated with broadly planar surfaces that locally retain the shape of historical river meanders. Qy2r surfaces are up to 7 meters above modern Qycr deposits and are the most extensive river terraces in the valley. Qy2r sediments were deposited when the San Pedro River was a widespread, shallowly-flowing river system and are dominated by fine grained floodplain deposits. Dense mesquite bosque and tall grass is typically present on these surfaces except where historic plowing or grazing has taken place. These surfaces appear predominantly fine grained at the surface due in part to the input of organic matter and windblown dust deposition but are composed of interfingering coarse sandy to pebbly braided channel and fine sand to silty river floodplain deposits. Where Qy2r deposits are moderately to deeply incised they are not subject to inundation by river floods, but they may be flood-prone in areas with less channel incision. Qy2r deposits are subject to catastrophic bank failure due to undercutting and lateral erosion during flow events. Distal piedmont fan deposits (Qy2 ,Qyaf, and Qys) onlap onto Qy2r deposits although an interfingering relationship likely exits in the subsurface. Qy2r Late Pleistocene river terrace deposits - River terrace deposits found on scattered hilltops along the San Pedro River. Qi3r deposits are thin, (<2 m) and composed of rounded to well-rounded pebbles, cobbles, and finer-grained sediment. Qi3r soils are moderately developed, with orange to reddish brown clay loam to light clay argillic horizons. Qi3r Middle to late Pleistocene river terrace deposits - River terrace deposits found on scattered hilltops along the San Pedro River. Qi2r surfaces are slightly higher than Qi3r surfaces. Qi2r deposits are thin, (< 2 m) and composed of rounded to well-rounded boulders, cobbles, and pebbles, with finer-grained sediments. Qi2r soils contain reddened, clay argillic horizons, with obvious clay skins and subangular blocky structure. Qi2r Piedmont alluvium and surficial deposits Modern stream channel deposits - Active channel deposits composed of moderately-sorted sand and pebbles with some cobbles in the lower piedmont areas to very poorly-sorted sand, pebbles, and cobbles with some boulders in the upper piedmont areas. Channels are generally incised less than 1 m below adjacent Holocene terraces and alluvial fans, but locally incision may be as much as 2 m. Channel morphologies generally consist of a single thread high flow channel or multi-threaded low flow channels with gravel bars. Channels are extremely flood prone and are subject to deep, high velocity flows in moderate to large flow events, and severe lateral bank erosion. Qyc Latest Holocene alluvium - Recently active piedmont alluvium located primarily along active drainages including floodplain, low-lying terrace, and overflow channels. Qy3 deposits are composed of unconsolidated to very weakly consolidated silty to cobbly deposits and exhibit greater vegetation than Qyc deposits. These deposits generally exhibit bar and swale microtopography and are susceptible to inundation during moderate to extreme flow conditions when channel flow exceeds capacity. Soil development is generally absent or incipient on Qy3 deposits which exhibit pale buff to light brown (10 YR) surface coloration. Qy3 Late Holocene alluvium, active fan deposits - Qyaf deposits consist of recently-active to active alluvial fan deposits in the San Pedro valley. These deposits have distributary drainage patterns and are extremely prone to flooding and channel migration. Sediments are unconsolidated and consist of very poorly sorted sand to cobbles. Vegetation includes small mesquite trees, shrubby acacia, prickly pear, and medium creosote. Qyaf Late Holocene alluvium - Young deposits in low terraces, alluvial fans, and small channels that are part of the modern drainage system. Includes Qyc and Qy3 where not mapped separately. In upper piedmont areas, channel sediment is generally poorly to very poorly-sorted sand and pebbles, but may include cobbles and boulders; terrace and fan surfaces typically are mantled with sand and finer sediment. On lower piedmont areas, young deposits consist predominantly of moderately sorted sand and silt, with some pebbles and cobbles in channels. Channels generally are incised less than 1 m below adjacent terraces and fans, but locally incision may be as much as 2 m. Channels are flood prone and may be subject to deep, high velocity flows in moderate to large flow events. Potential lateral bank erosion is severe. Channel morphologies generally consist of a single-thread high flow channel or multi-threaded low flow channels with gravel bars adjacent to low flow channels. Flood flows may significantly change channel morphology and flow paths. Downstream-branching distributary channel patterns - small, discontinuous, well-defined channels alternating with broad expansion reaches where channels are very small and poorly defined - are associated with young alluvial fans generally found along the base of the piedmont. Local relief varies from fairly smooth channel bottoms to undulating bar-and-swale topography that is characteristic of coarser deposits. Terraces have planar surfaces, but small channels are common. Soil development associated with Qy2 deposits is weak. Qy2 deposits range from unaltered fluvial sediments to weakly developed soil. Qy2 Older Holocene alluvium - Middle to older Holocene terrace deposits are found at scattered locations along incised drainages throughout the Whetstone piedmont. Qy1 surfaces are higher and less subject to inundation than adjacent Qy2 surfaces, and are generally planar. Local surface relief may be up to 1 m where gravel bars are present, but is typically much less. Qy1 surfaces are 1 to 2 m above adjacent active channels. Surfaces typically are sandy but locally have unvarnished open fine gravel lags. Qy1 soils typically are weakly developed, with some soil structure but little clay and no calcium carbonate accumulation. Yellow brown (10YR) soil color is similar to original fluvial deposits. Qy1 Fine-grained Holocene alluvium derived from the St. David Formation - Unit Qys consists of thin to moderate (< 3 m), fine-grain Holocene alluvium derived from, and overlying, the St. David Formation (Tsd). It is composed mostly of silts and clays with color reflecting that of the parent material. Qys is typically found in fans at the base of Tsd outcrops along the edges of the piedmont. Qys Undifferentiated Holocene alluvium - Includes Qyc, Qy2, and Qy1 deposits. On the upper piedmonts, unit Qy consists of smaller incised drainages where, at this scale, it was not possible to map surfaces separately. At the lower margin of the piedmont, unit Qy consists of young alluvial fans deposited by piedmont tributary streams interbedded with San Pedro River floodplain deposts (unit Qyr). Qy Holocene to late Pleistocene alluvial fan and terrace deposits - Abandoned stream terraces and fans on the upper piedmont, isolated from the active channels by 1 to 3 m. Unit Qyi consists of thin (<1 m) young alluvium, derived mainly from granite, over beveled Qi2 and Qi1 surfaces. The older units are exposed in patches on ridges and in cut banks of washes. Qyi surfaces are smooth to broadly rounded and are yellow brown (10YR 4/6) to brown (7.5Y 4/4). Qyi soils have weak subangular blocky structure with no to moderate clay accumulation and no carbonate accumulation to stage I+ (see Machette, 1985, for description of stages of calcium carbonate accumulation in soils). Qyi Late Pleistocene alluvial fan and terrace deposits - Moderately dissected relict terraces and alluvial fans which commonly flank active channel valley walls near the mountain front. Qi3 deposits are derived mainly from granite and consist of pebbles and finer-grained sediment. Qi3 soils are moderately developed, orange to reddish brown (7.5YR) with light to moderate clay accumulation and no carbonate accumulation to stage I+. Qi3 has a weak desert pavement with no interlocking of the clasts. Qi3 Middle to late Pleistocene alluvial fan and terrace deposits - Moderately to deeply dissected relict alluvial fans and terraces. Qi2 surfaces are drained by well-developed, moderately to deeply incised tributary channel networks; channels are typically several meters below adjacent Qi2 surfaces. Well preserved, planar Qi2 surfaces are smooth with scattered pebble lags. More eroded, rounded Qi2 surfaces are characterized by scattered cobble lags with moderate to strong varnish and broad ridge-like topography. Qi2 deposits are composed of grüss with isolated conglomerate lenses composed of metasediments, limestones, and granite cobbles. Soils typically contain reddened (5YR-2.5YR) clay argillic horizons, with obvious clay skins and subangular blocky structure. Carbonate accumulation is stage II to III and occurs mainly along root zones. Unit Qi2 covers much of the Whetstone piedmont. Qi2 Early to middle Pleistocene alluvial fan and terrace deposits - Deeply dissected relict alluvial fans found only on the upper piedmont. Qi1 surfaces form broadly rounded ridges that are higher than adjacent Qi2 surfaces. Qi1 soils are strongly developed with a distinct dark red (5-2.5 YR), heavy clay argillic horizon and subangular blocky to prismatic structure. Carbonate accumulations are 1-2 m thick and range from stage III - V. Qi1 Late Pliocene to early Pleistocene fan gravel - coarse, moderately to well-consolidated gravelly deposits capping high ridges. QTa Quaternary to Tertiary Basin Fill alluvium Pliocene-Pleistocene conglomerate - massive to crudely stratified, poorly bedded conglomerate interpreted as alluvial fan deposits shed into San Pedro River Valley from flanking bedrock hills and mountains. QTc Pliocene to Pleistocene conglomerate and sandstone - tan to reddish, coarse, poorly sorted, tabular to massive sandstone, conglomeratic sandstone and conglomerate. QTcs St. David Formation, undifferentiated (Pliocene) - Undivided lower and middle St. David Formation. Tsd Middle member of the St. David Formation - Middle St. David Formation consists of red and green mudstone, marl, tuff, lacustrine limestones, and fine to medium grained sandstones. The contact between the middle and lower St. David units is subtle and approximate. This unit is equivalent to units QTsp and QTsl in the St. David quandrangle. Tsdm Lower member of the St. David Formation - Lower St. David Formation consists of red mudstones and fine grained sandstones with nodular and fibrous gypsum located in the mudstones. This unit is equivalent to unit Ts in the Galleta Flat East quadrangle and unti Tsp in the St David quadrangle. Tsdl Bedrock units Bisbee Group, undifferentiated (Cretaceous to Jurassic) - A heterogeneous sequence of greenish-gray to purple, thin- to mediumbedded, argillaceous, feldspathic quartz sandstone, siltstone, mudstone and minor thin-bedded limestone (Skotnicki, 2001a and b). Hematite pseudomorphs after pyrite (2-8 mm) are common in the sandstones which are typically cross-stratified, and locally calcareous. Limestone beds, typically <20 cm thick, are laminated (locally with syndepositional folds) micrite with <2% silt to fine-grained quartz and feldspar sand grains and sparse gypsum pseudomorphs. Sparse medium- to thickbedded pebble-cobble conglomerate beds are also present containing clasts of pink quartzite, chert, and limestone. KJs Muscovite leucogranite (Mesoproterozoic) - Medium-grained, equigranular, but locally coarse-grained with sparse potassium feldspar, porphyritic leucogranite containing 5-20% muscovite. Ygm Granite (Mesoproterozoic) - Medium- to coarse-grained, potassium feldspar porphyritic (up to 3 cm) granite with between 5-12% biotite. Yg Disturbed ground - Areas where human activity has obscured the underlying geology. Typically these are borrow pits. d San Pedro River alluvium Active river channel deposits - Deposits are dominantly unconsolidated, very poorly sorted sandy to cobbly beds exhibiting bar and swale microtopography but can range from fine silty beds to coarse gravelly bars in meandering reaches based on position within the channel. Clasts are typically well-rounded but may be angular to sub-angular. Qycr deposits are typically unvegetated to lightly vegetated and exhibit no soil development. Qycr deposits are entrenched from 30 cm to 5 meters or more below adjacent early historical floodplain deposits depending on location, geomorphic relationship, and local channel conditions. Although much of the San Pedro was a perennial stream historically, some sections are dry or marshy at the surface during much of the year. These deposits are the first to become submerged during moderate to extreme flow events and can be subject to deep, high velocity flow and lateral bank erosion. Extent of channel deposit and exposed bedrock varies and shifts with significant flooding. Qycr
Transcript of DGM-34 (Benson), v. 2.0 GEOLOGIC MAP OF THE BENSON...
Arizona Geological Survey DGM-34 (Benson), v. 3.0
Arizona Geological SurveyDGM-34 (Benson), v. 2.0
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Qy1
Qyi
Qys
QTcs
Qi2
Qi3r
Qi2
Qy
Yg
Qy1
Qi2
QyQi3r
Qy1
Qy1
Qy
Qy
Qy1
Tsd
Qy1
Qyi
Qys
Qy1
QTcs
Qy
Qi3r
QTcs
Qy1
Qyc
Qy
Qy2
Qi2
Tsd
Qy1
Qi3r
KJs
Qy2r
Qy1
QTc
Qy3Qy
Qyi
Qi3
Qi2Qi2
Qy2
Qi3r
Qi2
Qy
Qy2
Qy2
Qy2
QyQi3
Qi2
Qi2
QTcs
Qi1
Qy
Qyi
Tsd
Qy
Qy1
Tsdl
Tsd
KJs
Tsd
Qys
Qy1
Qy1
Tsd
Qys
Qi2
Qy1
Qi3r
Yg
Qy2
Qy
Qy2
Qy2
Qy
Qi1
Qy1
Qy2
Qy2
Tsd
Tsd
Qy
Qi3r
Qy
Qy1
Qi3
Tsd
Tsd
Qy
Qi2
Qi3
Qi1
Qyc
Tsd
Qyc
Tsd
Qy1 Qy2
Qy1
Ygm
QTcs
Qy1
Qi3r
Qi2
Tsd
Qy1
Qyi
Qi2
Qy2
Yg
Qyi
Tsd
Qi2
Tsd
Qi1
TsdQy
Qy1
Tsd
Qy
Tsd
Qi1
Qyi
Qy
Qyi
Qy
Qyi
Qi3
Qy1
Qi2
Tsd
Qy
Qi3
Qy2
Qy1
Tsdl
Tsd
Qy1
Qy1
Qy
QTcs
Qy2
Qi2
Qi3
Tsd
Qi3r
Qy2
Qi3r
Qi2r
QTc
QTcs
Tsd
Qy1
Qi2r
Qy1
Qy
Tsd
Qi2
Qy
Qy2
Qy
Qy2
Qi1
Qi2
Tsd
Qy
Qi2
Qy1
Qyi
Tsd
Qyi
Qi2
Qy1
Qi1
Qy
Qyi
Qys
Qy
Qi3
Qy1
Yg
Qyi
Qi3
Qy1
Qy1
QTa
QTa
QTa QTa QTa
Qy3r
559000 560000 561000 562000 563000 564000 565000 566000 567000 568000 569000 570000 571000
559000 560000 561000 562000 563000 564000 565000 566000 567000 568000 569000 570000 571000
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110°15'
110°15'
110°17'30"
110°17'30"
110°20'
110°20'
110°22'30"
110°22'30"
32°0' 32
°0'
31°57
'30"
31°57
'30"
31°55
'
31°55
'
31°52
'30"
31°52
'30"
2006 MAGNETIC NORTHDECLINATION
^
10½º
MN
Arizona Geological Survey1955 E 6th St Tucson, AZ 85721
(520) 621-2352www.azgs.arizona.edu
Topographic base from USGS 1:24,000 scale quadrangle series.North American Datum of 1983 (NAD83). Projection and 1,000-meter grid: Universal Transverse Mercator, zone 12. 1 0 10.5 Miles
1 0 10.5 Kilometers
0 1000 2000 3000 4000 5000 Feet
SCALE 1:24,000
CONTOUR INTERVAL 20 FEET
Map SymbolsFaults
fault, accuratefault, concealedfault, concealed, hypothetical@@
Contactsaccurate contactapproximate contactscratch contact concealed contact
! ! !
Youberg andShipmanSkotnicki Ferguson
Youbergand
Skotnicki
Cook and Youberg
110°15'
110°15'
110°17'30"
110°17'30"
110°20'
110°20'
110°22'30"
110°22'30"
32°0' 32°0'
31°57
'30"
31°57
'30"
31°55
'
31°55
'
31°52
'30"
31°52
'30"
Mapping Responsibility
GEOLOGIC MAP OF THE BENSON 7½' QUADRANGLE, COCHISE COUNTY,
ARIZONAAnn Youberg, Steven J. Skotnicki, Charles A. Ferguson,
Joseph P. Cook and Todd C. ShipmanArizona Geological Survey Digital Geologic Map 34
(DGM-34), version 3.0September 2019
1:24,000 scale
BISBEE
BENSON
COCHISE
DOUGLAS
TOMBSTONE
SIERRA VISTA
§̈¦10
£¤80
£¤92
£¤186
£¤181
£¤82
£¤90
£¤191
109°0'
109°15'
109°15'
109°30'
109°30'
109°45'
109°45'
110°0'
110°0'
110°15'
110°15'
110°30'
110°30'
32°15
'
32°15
'
32°0' 32°0'
31°45
'
31°45
'
31°30
'
31°30
'Benson 7.5' Quadrangle is located in western Cochise County.
LAND
MESCAL BENSON SAINT DAVID
APACHE PEAK MCGREW SPRING
SAN PEDRO RANCHGALLETA FLAT EASTGALLETA FLAT WEST
HABERSTOCK HILL
110°7.5'
110°7.5'
110°15'
110°15'
110°22.5'
110°22.5'
110°30'
110°30'
32° 32°
31°52
.5'
31°52
.5'
Adjoining 7.5' QuadranglesLocation Index MapQuadrangle Location Shown in Blue
Cochise CountyMapped Area Shown in Blue
Unit Correlation
Citation for this map: Youberg, A., Skotnicki, S.J., Ferguson, C.A., Cook, J.P., and Shipman, T.C.,
2019, Geologic Map of the Benson 7½' Quadrangle, Cochise County, Arizona: Arizona Geological Survey Digital Geologic Map 34 (DGM-34),
version 3.0, 1 sheet, layout scale 1:24,000.(also available in Adobe pdf format on CD-ROM)
Not to be reproduced for commercial purposesResearch supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program, under USGS award #02HQAG0016. The views and conclusions contained in this document are those
of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. government. The Arizona Department of Water Resources
provided funding for map revisions depicted in DGM-34, version 2.0.
Acknowledgements:Genevieve Pearthree was the cartographer for DGM-34, version 2.0.
Map Unit DescriptionsOther units
Plowed areas - historically or actively plowed fields, irrigated pastures, and other lightly disturbed ground.
Historical river terrace deposits - Terrace deposits that occupy elevations from 1 to 2 meters above Qycr deposits and are inset below the pre-incision historical floodplain. These surfaces are generally planar but exhibit bar and swale microtopography. Although no soil development is present, dense grasses and small mesquite trees abound. Sediments composing these deposits are poorly sorted silt, sand, pebbles and cobbles. Pebbles and cobbles are well-rounded to sub-angular. Trough crossbedding, ripple marks, and stacked channel deposits viewable in cross-section indicate deposition in a low to moderate energy braided stream environment. These deposits are prone to flooding during extreme flow events, and undercutting and rapid erosion of Qy3r surfaces is possible during lower flow events.
Qy3r
Latest Holocene to historical river terrace deposits - Deposits associated with the floodplain that existed prior to the early historical entrenchment of the San Pedro River (Hereford, 1993; Huckleberry, 1996; Wood, 1997). Qy2r deposits are associated with broadly planar surfaces that locally retain the shape of historical river meanders. Qy2r surfaces are up to 7 meters above modern Qycr deposits and are the most extensive river terraces in the valley. Qy2r sediments were deposited when the San Pedro River was a widespread, shallowly-flowing river system and are dominated by fine grained floodplain deposits. Dense mesquite bosque and tall grass is typically present on these surfaces except where historic plowing or grazing has taken place. These surfaces appear predominantly fine grained at the surface due in part to the input of organic matter and windblown dust deposition but are composed of interfingering coarse sandy to pebbly braided channel and fine sand to silty river floodplain deposits. Where Qy2r deposits are moderately to deeply incised they are not subject to inundation by river floods, but they may be flood-prone in areas with less channel incision. Qy2r deposits are subject to catastrophic bank failure due to undercutting and lateral erosion during flow events. Distal piedmont fan deposits (Qy2 ,Qyaf, and Qys) onlap onto Qy2r deposits although an interfingering relationship likely exits in the subsurface.
Qy2r
Late Pleistocene river terrace deposits - River terrace deposits found on scattered hilltops along the San Pedro River. Qi3r deposits are thin, (<2 m) and composed of rounded to well-rounded pebbles, cobbles, and finer-grained sediment. Qi3r soils are moderately developed, with orange to reddish brown clay loam to light clay argillic horizons.
Qi3r
Middle to late Pleistocene river terrace deposits - River terrace deposits found on scattered hilltops along the San Pedro River. Qi2r surfaces are slightly higher than Qi3r surfaces. Qi2r deposits are thin, (< 2 m) and composed of rounded to well-rounded boulders, cobbles, and pebbles, with finer-grained sediments. Qi2r soils contain reddened, clay argillic horizons, with obvious clay skins and subangular blocky structure.
Qi2r
Piedmont alluvium and surficial deposits
Modern stream channel deposits - Active channel deposits composed of moderately-sorted sand and pebbles with some cobbles in the lower piedmont areas to very poorly-sorted sand, pebbles, and cobbles with some boulders in the upper piedmont areas. Channels are generally incised less than 1 m below adjacent Holocene terraces and alluvial fans, but locally incision may be as much as 2 m. Channel morphologies generally consist of a single thread high flow channel or multi-threaded low flow channels with gravel bars. Channels are extremely flood prone and are subject to deep, high velocity flows in moderate to large flow events, and severe lateral bank erosion.
Qyc
Latest Holocene alluvium - Recently active piedmont alluvium located primarily along active drainages including floodplain, low-lying terrace, and overflow channels. Qy3 deposits are composed of unconsolidated to very weakly consolidated silty to cobbly deposits and exhibit greater vegetation than Qyc deposits. These deposits generally exhibit bar and swale microtopography and are susceptible to inundation during moderate to extreme flow conditions when channel flow exceeds capacity. Soil development is generally absent or incipient on Qy3 deposits which exhibit pale buff to light brown (10 YR) surface coloration.
Qy3
Late Holocene alluvium, active fan deposits - Qyaf deposits consist of recently-active to active alluvial fan deposits in the San Pedro valley. These deposits have distributary drainage patterns and are extremely prone to flooding and channel migration. Sediments are unconsolidated and consist of very poorly sorted sand to cobbles. Vegetation includes small mesquite trees, shrubby acacia, prickly pear, and medium creosote.
Qyaf
Late Holocene alluvium - Young deposits in low terraces, alluvial fans, and small channels that are part of the modern drainage system. Includes Qyc and Qy3 where not mapped separately. In upper piedmont areas, channel sediment is generally poorly to very poorly-sorted sand and pebbles, but may include cobbles and boulders; terrace and fan surfaces typically are mantled with sand and finer sediment. On lower piedmont areas, young deposits consist predominantly of moderately sorted sand and silt, with some pebbles and cobbles in channels. Channels generally are incised less than 1 m below adjacent terraces and fans, but locally incision may be as much as 2 m. Channels are flood prone and may be subject to deep, high velocity flows in moderate to large flow events. Potential lateral bank erosion is severe. Channel morphologies generally consist of a single-thread high flow channel or multi-threaded low flow channels with gravel bars adjacent to low flow channels. Flood flows may significantly change channel morphology and flow paths. Downstream-branching distributary channel patterns - small, discontinuous, well-defined channels alternating with broad expansion reaches where channels are very small and poorly defined - are associated with young alluvial fans generally found along the base of the piedmont. Local relief varies from fairly smooth channel bottoms to undulating bar-and-swale topography that is characteristic of coarser deposits. Terraces have planar surfaces, but small channels are common. Soil development associated with Qy2 deposits is weak. Qy2 deposits range from unaltered fluvial sediments to weakly developed soil.
Qy2
Older Holocene alluvium - Middle to older Holocene terrace deposits are found at scattered locations along incised drainages throughout the Whetstone piedmont. Qy1 surfaces are higher and less subject to inundation than adjacent Qy2 surfaces, and are generally planar. Local surface relief may be up to 1 m where gravel bars are present, but is typically much less. Qy1 surfaces are 1 to 2 m above adjacent active channels. Surfaces typically are sandy but locally have unvarnished open fine gravel lags. Qy1 soils typically are weakly developed, with some soil structure but little clay and no calcium carbonate accumulation. Yellow brown (10YR) soil color is similar to original fluvial deposits.
Qy1
Fine-grained Holocene alluvium derived from the St. David Formation - Unit Qys consists of thin to moderate (< 3 m), fine-grain Holocene alluvium derived from, and overlying, the St. David Formation (Tsd). It is composed mostly of silts and clays with color reflecting that of the parent material. Qys is typically found in fans at the base of Tsd outcrops along the edges of the piedmont.
Qys
Undifferentiated Holocene alluvium - Includes Qyc, Qy2, and Qy1 deposits. On the upper piedmonts, unit Qy consists of smaller incised drainages where, at this scale, it was not possible to map surfaces separately. At the lower margin of the piedmont, unit Qy consists of young alluvial fans deposited by piedmont tributary streams interbedded with San Pedro River floodplain deposts (unit Qyr).
Qy
Holocene to late Pleistocene alluvial fan and terrace deposits - Abandoned stream terraces and fans on the upper piedmont, isolated from the active channels by 1 to 3 m. Unit Qyi consists of thin (<1 m) young alluvium, derived mainly from granite, over beveled Qi2 and Qi1 surfaces. The older units are exposed in patches on ridges and in cut banks of washes. Qyi surfaces are smooth to broadly rounded and are yellow brown (10YR 4/6) to brown (7.5Y 4/4). Qyi soils have weak subangular blocky structure with no to moderate clay accumulation and no carbonate accumulation to stage I+ (see Machette, 1985, for description of stages of calcium carbonate accumulation in soils).
Qyi
Late Pleistocene alluvial fan and terrace deposits - Moderately dissected relict terraces and alluvial fans which commonly flank active channel valley walls near the mountain front. Qi3 deposits are derived mainly from granite and consist of pebbles and finer-grained sediment. Qi3 soils are moderately developed, orange to reddish brown (7.5YR) with light to moderate clay accumulation and no carbonate accumulation to stage I+. Qi3 has a weak desert pavement with no interlocking of the clasts.
Qi3
Middle to late Pleistocene alluvial fan and terrace deposits - Moderately to deeply dissected relict alluvial fans and terraces. Qi2 surfaces are drained by well-developed, moderately to deeply incised tributary channel networks; channels are typically several meters below adjacent Qi2 surfaces. Well preserved, planar Qi2 surfaces are smooth with scattered pebble lags. More eroded, rounded Qi2 surfaces are characterized by scattered cobble lags with moderate to strong varnish and broad ridge-like topography. Qi2 deposits are composed of grüss with isolated conglomerate lenses composed of metasediments, limestones, and granite cobbles. Soils typically contain reddened (5YR-2.5YR) clay argillic horizons, with obvious clay skins and subangular blocky structure. Carbonate accumulation is stage II to III and occurs mainly along root zones. Unit Qi2 covers much of the Whetstone piedmont.
Qi2
Early to middle Pleistocene alluvial fan and terrace deposits - Deeply dissected relict alluvial fans found only on the upper piedmont. Qi1 surfaces form broadly rounded ridges that are higher than adjacent Qi2 surfaces. Qi1 soils are strongly developed with a distinct dark red (5-2.5 YR), heavy clay argillic horizon and subangular blocky to prismatic structure. Carbonate accumulations are 1-2 m thick and range from stage III - V.
Qi1
Late Pliocene to early Pleistocene fan gravel - coarse, moderately to well-consolidated gravelly deposits capping high ridges.QTa
Quaternary to Tertiary Basin Fill alluvium
Pliocene-Pleistocene conglomerate - massive to crudely stratified, poorly bedded conglomerate interpreted as alluvial fan deposits shed into San Pedro River Valley from flanking bedrock hills and mountains.QTc
Pliocene to Pleistocene conglomerate and sandstone - tan to reddish, coarse, poorly sorted, tabular to massive sandstone, conglomeratic sandstone and conglomerate.QTcs
St. David Formation, undifferentiated (Pliocene) - Undivided lower and middle St. David Formation.Tsd
Middle member of the St. David Formation - Middle St. David Formation consists of red and green mudstone, marl, tuff, lacustrine limestones, and fine to medium grained sandstones. The contact between the middle and lower St. David units is subtle and approximate. This unit is equivalent to units QTsp and QTsl in the St. David quandrangle.
Tsdm
Lower member of the St. David Formation - Lower St. David Formation consists of red mudstones and fine grained sandstones with nodular and fibrous gypsum located in the mudstones. This unit is equivalent to unit Ts in the Galleta Flat East quadrangle and unti Tsp in the St David quadrangle.Tsdl
Bedrock units
Bisbee Group, undifferentiated (Cretaceous to Jurassic) - A heterogeneous sequence of greenish-gray to purple, thin- to mediumbedded, argillaceous, feldspathic quartz sandstone, siltstone, mudstone and minor thin-bedded limestone (Skotnicki, 2001a and b). Hematite pseudomorphs after pyrite (2-8 mm) are common in the sandstones which are typically cross-stratified, and locally calcareous. Limestone beds, typically <20 cm thick, are laminated (locally with syndepositional folds) micrite with <2% silt to fine-grained quartz and feldspar sand grains and sparse gypsum pseudomorphs. Sparse medium- to thickbedded pebble-cobble conglomerate beds are also present containing clasts of pink quartzite, chert, and limestone.
KJs
Muscovite leucogranite (Mesoproterozoic) - Medium-grained, equigranular, but locally coarse-grained with sparse potassium feldspar, porphyritic leucogranite containing5-20% muscovite.Ygm
Granite (Mesoproterozoic) - Medium- to coarse-grained, potassium feldspar porphyritic (up to 3 cm) granite with between 5-12% biotite.Yg
Disturbed ground - Areas where human activity has obscured the underlying geology. Typically these are borrow pits.d
San Pedro River alluvium
Active river channel deposits - Deposits are dominantly unconsolidated, very poorly sorted sandy to cobbly beds exhibiting bar and swale microtopography but can range from fine silty beds to coarse gravelly bars in meandering reaches based on position within the channel. Clasts are typically well-rounded but may be angular to sub-angular. Qycr deposits are typically unvegetated to lightly vegetated and exhibit no soil development. Qycr deposits are entrenched from 30 cm to 5 meters or more below adjacent early historical floodplain deposits depending on location, geomorphic relationship, and local channel conditions. Although much of the San Pedro was a perennial stream historically, some sections are dry or marshy at the surface during much of the year. These deposits are the first to become submerged during moderate to extreme flow events and can be subject to deep, high velocity flow and lateral bank erosion. Extent of channel deposit and exposed bedrock varies and shifts with significant flooding.
Qycr
